Previously, it has been demonstrated that the chlorobutanol induced shortening of the Limulus ventral eye photoreceptor potential latent period was intensely dependent, virtually disappearing at stimulating intensities approaching threshold. This finding suggested the existence of a chlorobutanol sensitive component of the latent period. The result described above also suggests that the effect of chlorobutanol on receptor potential latency may be dependent on the state of light adaptation. The results of recent experiments demonstrated that the latency shortening effect of chlorobutanol is maximal in dark adapted photoreceptors and, at the highest useable levels of light adaptation, the latencies of sense cells exposed to chlorobutanol are about equal to those exposed only to sea water. This finding suggests that light and chlorobutanol may accelerate the same component of the latent period.